The technique for chemically analyzing living tissue through nuclear magnetic resonance (NMR) phenomena is well known and essentially includes locating a tissue sample and a transmitter/receiver probe within a strong magnetic field, and using the probe to excite the tissue with RF energy and measure the frequency and strength of the RF energy absorbed or emitted by the tissue. One type of probe used in such magnetic resonance analysis includes multiple antenna surface-coil probes generally comprised of a larger outer circular coil employed as a transmitter and a smaller inner circular coil employed as a receiver. These probes are useful in proton decoupled carbon-13 NMR, DEPTH pulse sequence spatial focusing, Fourier series spatial windowing and one-dimensional rotating frame Zeugmatography. For simplicity, reference will be made solely to the multiple antenna surface-coil NMR probes in one of these applications while it shall be understood that other applications are similarly suitable for similar types of probe designs.
With multiple antenna surface-coil NMR probes, the primary technical problem in the prior art has been the electromagnetic coupling between the two coils. The magnetic field produced by the RF energy driving the transmitter or larger coil has a tendency to induce an RF current in the smaller receiver coil that generates an opposing electromagnetic field which distorts the distribution of RF energy, the magnetic field pattern, of the larger transmitter coil. In a similar, although much less marked manner, the small receiver coil can be thought of as also inducing current in the large transmitter coil. This effect is especially severe when both coils are tuned to the same operating frequency and are co-axially oriented. While this coupling between coils may be measured and adjusted for, it is undesirable in that it generally dramatically degrades the frequency and impedance tuning of the transmitter/receiver electrical circuit, perturbs the desired characteristic of each antenna, complicates the analysis required to interpret the results, and also is an unwanted variable which reduces the accuracy of the data collected.
In the prior art, a number of methods have been developed and utilized for reducing the coupling between coils in a multiple antenna surface-coil probe. These methods typically employ quarter wavelength co-axial transmission lines and/or crossed diodes. Although advances in such coil-to-coil decoupling techniques have yielded improved isolation and workable co-axial surface-coil probe designs, operational problems remain. Non-idealities of transmission lines and diodes lower the antenna circuit quality factor (Q) of the coils. Furthermore, proper adjustment of quarter wavelength cable length and placement in the circuit are non-trivial due to the cable size and required cable change for change in operating frequency. With crossed diodes, separate tuning and matching of each coil requires (ideally) sufficient RF current to short circuit the diodes.
To solve these and other problems in the prior art, the inventors have succeeded in designing and developing a co-axial multiple antenna surfacecoil NMR probe which utilizes typically a larger circular loop for the transmitter coil and a smaller receiver coil consisting of not one but two circular loop elements wound in the opposite direction, the elements being oriented generally symmetrically about the transmitter loop. Therefore, with the transmitter coil located midway between the two opposed loops comprising the receiver coil, the transmitter magnetic field can be thought of as inducing currents in the two loops of the receiver coil of the same strength but in opposite directions in the single conductor. This results in a net induced current in the receiver coil of substantially zero such that there is no magnetic field produced corresponding to an induced current to disturb the field distribution pattern of the transmitter coil. Furthermore, the second loop added to the typical single loop receiver coil does not enter into the detection operation of the receiver coil in that the second receiver coil loop is sufficiently far away from the sample that it can be ignored. In other words, the second receiver coil loop is beyond the sensitive volume of the sample region of the first receiver coil loop. Thus, the receiver antenna of the present invention becomes essentially equivalent to the single loop receiver antenna of the prior art that is typically positioned adjacent to the sample. The inventors have constructed a probe of the present invention which achieves isolation of greater than 40 db between the transmit and receive antennas, and believe that isolation of upwards of 50 db may be readily attained.
While there are many advantages and features of the present invention over the prior art, some of these include the fact that the inventors' device relies simply on the geometry of the receiver coil to achieve decoupling. With such an arrangement, standard, proven surface-coil designs may be readily utilized without alteration and without the difficulties experienced in the prior art of utilizing particular frequency dependent transmission line filters or diode elements. By utilizing standard proven surface-coil designs, well defined fields are guaranteed to be produced so as to optimize the results attainable from NMR techniques utilizing the present probe. Well defined fields are generally considered as being fields over the region of interest which are homogeneous, exhibit a linear gradient, or which produce typical and well documented field patterns for well known surface-coil type designs, e.g., circular or rectangular coils. Also, the inventors' design is frequency independent which provides maximum versatility for a probe.
Other embodiments of the present invention are also disclosed, including embodiments demonstrating asymmetrical receiver coil arrangements, dual element transmitter coils, and other non-orthogonal arrangements which also achieve the purposes of the present invention, i.e., that of decoupling the transmitter coil from the receiver coil in the NMR probe.
While the pricipal advantages and features of the present invention have been described above, a greater understanding and appreciation for the objects of the present invention may be attained by referring to the drawings and description of the preferred embodiment which follow.